1. Field of the Invention
This invention relates to DC-DC, AC-DC, DC-AC and AC-AC power converters. Power is transferred in the circuit by using switches, which turn on under zero voltage conditions.
2. Description of the Prior Art
There is a continuous industry demand for increasing power density and efficiency in power converters. A method for increasing the power transfer through a converter is to increase the switching frequency in order to minimize the size of inductors and capacitors. However, in conventional square wave power conversion topologies, an increase in frequency means an increase in switching losses and of the electromagnetic interference (EMI) level. Several power conversion techniques were developed such as resonant and quasi-resonant topologies, which significantly decrease the switching losses. But these power conversion techniques bring their own set of problems, such as frequency modulation, large current or voltage stresses on the switching elements, and in some cases circulating currents, which affects the efficiency. As a result these topologies did not receive much acceptance from the power supply industry.
For high output power applications half bridge and full bridge converters are the most suitable. However, conventional half bridge and full bridge converters exhibit several shortcomings which makes them incompatible with very high frequency operation, especially in high input voltage applications. One of the problems associated with conventional half and full bridge converters is the switching losses, which are proportional with the operation frequency. Another shortcoming associated with conventional technologies is the fact that the leakage inductance of the transformer is not used or totally recycled. Most of the time the energy associated with the leakage inductance is dissipated in snubbing circuits. A hard switching technique, associated with conventional technology, leads to an increased level of EMI. General Electric has introduced a "new" full bridge zero voltage switching technology several years ago, known in industry as a "phase shifted full bridge converter". Unfortunately the control technique for this technology is very sophisticated and expensive. See, Steigerwald et al., "Full-Bridge Lossless Switching Converter," U.S. Pat. No. 4,864,479 (1989).
The half and full bridge converter which is the subject of this disclosure combines simplicity with the low cost and high performance. Zero voltage switching conditions are created for the switches, switching losses eliminated, and very high frequency operation allowed.